Chemical milling is that process whereby metal or metal alloys are removed in a controlled process from a metal surface by selective chemical etching in either an acid or alkaline bath. It is especially useful for removing minute amounts of metal from sheet components, particularly those that are employed in the aerospace industry. While aluminum or its alloys are the primary compounds that are chemically milled, any acid or base soluble metal can be milled. Thus, in addition to aluminum alloys, beryllium, magnesium, titanium, copper, tantalum, uranium, and various steel and stainless steel alloys are also millable, as well as refractory metals, particularly alloys of molybdenum, tungsten, niobium (columbium) and zirconium.
The chemical reactions involved in milling aluminum or alloys of aluminum are as follows: EQU 2A1 + 2NaOH + 2H.sub.2 O .fwdarw. 2NaA1O.sub.2 + 3H.sub.2
and EQU NaA1O.sub.2 + H.sub.2 O .revreaction. A1(OH).sub.3 + NaOH
There are three important considerations in performing chemical milling generally, and particularly as applied to milling aluminum. The first is controlling the working conditions in order to obtain, among other things, a proper etch rate range and a smooth surface; second, identifying chemicals or techniques that will prolong the etching power, known by those skilled in the art as etch rate of the etch solution. The third, intertwined with the first two, is removal of by-products produced in the process which retard or reduce the activity of the solution.
The production of NaAlO.sub.2 during the etching process is primarily responsible for the loss of etching power. Under normal operating conditions NaAlO.sub.2 builds up, and forms a rock-like hydrate on the walls of the etching tank. Presently there are several procedures for reducing the level of NaAlO.sub.2 in the etching solution, particularly the addition of chelating agents that prevent the precipitation of NaAlO2 as described in U.S. Pat. No. 3,557,000. Further, it is known (U.S. Pat. No. 4,417,949, Inventor Lindner) that the addition of NaNO.sub.3 to the etch solution reduces the formation of NaAlO.sub.2. Procedures for removing the NaAlO.sub.2 as Al(OH).sub.3 have also been detail (U.S. Pat. No. 4,372,805, Inventors Takahashi and Hamamura, U.S. Pat. No. 4,136,026, Inventors Meyer, Johnson, Wodehorse).
The second major concern of the chemical milling industry is the development of procedures for disposing of the solution when it has expired beyond the point of being useful. The main problem associated with disposing of depleted solutions is getting rid of the dissolved aluminum, sulfides and organic compounds present in the solution, as well as other chemicals. For the most part, suitable disposal of the chemical etch solutions remains an unsolved problem.
Indeed, the industry has not developed a satisfactory procedure for disposal of the etch solution, other than merely carting it off to a chemical dumpsite, or removing some of the dissolved aluminum. It has become very costly to dispose of these solutions because of the enormous volumes of solutions, and because of new government restrictions on the disposal of wastes.